The invention relates to a semipermeable membrane made from a homogeneously miscible polymer blend and to a process for its production.
Since the introduction of asymmetric membranes made from cellulose acetate by Loeb and Sourirajan (S. Sourirajan, Reverse Osmosis, Logos Press, London 1970) and made from hydrophobic polymers (U.S. Pat. No. 3,615,024), numerous membranes, in particular for separations of low-molecular weight and macromolecular components dissolved in water, have been developed and proposed, and their structure and suitability have been described in the literature (Desalination, 35 (1980), 5-20). They have also been successfully tested in industry or for medical purposes.
Many of the membranes described have particularly advantageous properties for achieving specific objectives. As a consequence of their chemical and physical structure, each of the individual membranes can only be optimally suitable for very specific separation problems. This gives rise to the basic need for always developing new membranes for new problems.
EP-A-0 082 433 gives a clear description of the advantages and disadvantages of already known membranes. Thus, there are, for example, hydrophilic, asymmetric membranes made from cellulose acetate which have satisfactory antiadsorptive properties, but which leave much to be desired with respect to their thermal and chemical resistance. Membranes made from polysulfones or similar polymers may have good thermal and chemical resistance. There is, however, a pronounced tendency in membranes of this type, due to the hydrophobic properties of the polymers employed, to adsorb dissolved substances, causing the membrane to become more or less blocked. Although the mixtures of polysulfone and polyvinylpyrrolidone described in EP-A-0 082 433 eliminate the disadvantage caused by the hydrophobic character of polysulfone, these mixtures are, however, sensitive to the influence of organic solvents. There are also problems when these membranes are used for the treatment of waste water, because so-called silicone defoamers which may be present in the waste water will block the membranes.
U.S. Pat. No. 4,051,300 describes mixtures of aromatic polyamides with polyvinylpyrrolidone. However, the polyamides are said to have a limited compatibility with the polyvinyl pyrrolidone. These membranes still need improving with respect to their hydrophilic character.
Hydrophilic character and simultaneous resistance to solvents are found in membranes of regenerated cellulose; however, these can be hydrolyzed relatively easily in acidic or alkaline media, and, in addition, they are easily attacked by microorganisms.